1. Field of the Invention
The present invention relates to a DC voltage converter for deriving a DC voltage with a high degree of accuracy from a DC power supply whose voltage is liable to vary, based on a so-called step-down voltage chopper principle and more particularly to a so-called step-down voltage chopper type DC-DC converter.
2. Description of the Prior Art
In the voltage converter of the type described above, an output voltage is limited only to a voltage lower than a power supply voltage, but this voltage converter has been widely used as a DC constant voltage power source in various electronic equipment and circuits, not only because its operation principle is simple but also because the converter is relatively simple in circuit arrangement. From a practical viewpoint, however, its operation performance and stability are not satisfactory. Furthermore, there is a problem that the circuit arrangement is not so simple to arrange as a constant voltage power source suitable for mass production, so that its cost is still expensive. The above and other problems encountered in the prior art step-down voltage chopper type DC-DC converter will be briefly described below with reference to FIGS. 1 and 2 for the sake of better understanding of the present invention.
In FIG. 1, a circuit portion surrounded by the dash-and-dotted lines represents a voltage converter of the type described above. The voltage Ei of a power source 1 is supplied to the converter as indicated by the broken lines, so that a constant voltage Eo is applied to a load 2. In the voltage converter as a load power source circuit inserted between the power supply 1 and the load 2, a switching transistor 3 and a reactor 4 are connected in series to each other. Thus, even if the power source voltage Ei varies, the voltage drop across the terminals of the reactor 4 compensates for a variation in the power source voltage Ei, so that the constant voltage Eo is applied across the load 2. On the output side, a capacitor 5 having a relatively large capacitance is connected in parallel with the load 2, so that the output voltage Eo is stabilized. Reference numeral 6 represents a so-called free wheeling diode which is rendered to be conductive when the switching transistor 3 is interrupted, so that the energy stored in the reactor 4 is supplied as a current to the load 2.
In FIG. 1, reference numeral 7 designates a reference power source for producing a reference voltage E to be used to set the output voltage level Eo. Two resistors 8a and 8b are connected in series to each other and the series connection of the resistors 8a and 8b is connected in parallel with the capacitor 5. A divided voltage Vo corresponding to the reference voltage E is derived from the junction between the resistors 8a and 8b which divide the output voltage Eo. Reference numeral 9a denotes a hysteresis circuit for controlling an oscillation condition of the whole circuit. An operational amplifier 9b which functions as a differential amplifier or a comparator produces a switching instruction signal SS, which is applied to the transistr 3. The hysteresis circuit 9a receives the switching instruction signal and the reference voltage E from the reference power source 7. The hysteresis circuit 9a generates a hysteresis voltage Vh which varies on both sides of the reference voltage E within a constant voltage range .DELTA.V in synchronism with the switching signal SS. The waveform of the voltage Vh is indicated by the broken line in FIG. 2.
The operational amplifier 9b receives both the hysteresis voltage Vh and the above-described divided voltage Vo to generate the switching instruction signal SS which turns on the transistor 3 when the hysteresis voltage Vh is higher than the divided voltage Vo, and turns off when the hysteresis voltage Vh is lower than the divided voltage Vo. As shown in the lower portion of FIG. 2, the hysteresis voltage Vh has a rectangular waveform, while the divided voltage Vo has a triangular waveform which rises when the transistor 3 is turned on, and falls when the transistor 3 is turned off. At an intersection between the voltage waveforms Vh and Vo, the output of the operational amplifier 9b is reversed and the switching instruction signal SS is generated to turn on or off the transistor 3.
If the output voltage Eo happens to rise above a normal level, the level of the divided voltage Vo shown in FIG. 2 rises as illustrated by the dash-and-dotted line, so that the intersection between the upper level of the hysteresis voltage Vh and the divided voltage Vo; that is, the time point at which the transistor 3 is turned off is advanced, and the time point at which the transistor 3 is turned on and which is determined in accordance with the lower level of the hysteresis voltage Vh and the divided voltage Vo is delayed. That is, when the output voltage Eo is higher than the normal level, the "ON" time of the transistor 3 is short and the "OFF" time is increased, so that the level of the output voltage Eo is decreased. Thus, it is apparent that when the output voltage Eo is lower than the normal level, the "ON" time of the transistor 3 is extended and the "OFF" time is shortened, so that the output voltage Eo is increased.
In this manner, the divided voltage Vo corresponding to the output voltage Eo is limited between the upper and lower level of the reference voltage E; that is, it is limited within the range .DELTA.V of the reference voltage E. As a result, even if the power voltage Ei varies, the output voltage Eo is maintained substantially at a constant voltage containing some ripple or pulsating component.
However, in the case of the above-mentioned conventional circuit, the hysteresis width .DELTA.V, that is, the above-mentioned voltage range derived from the hysteresis circuit 9a is likely to vary. As a result, the control characteristic of the circuit is likely to vary. More particularly, the hysteresis circuit 9a and the operational amplifier 9b establish a closed loop and the operational amplifier 9b having a high gain of the order of thousands or ten thousands is used in order to improve the control characteristics, so that the hysteresis width .DELTA.V tends to vary in response to a slight variation in the control characteristics. As a consequence, the constant-voltage-accuracy of the output voltage tends to be affected. As is well known in the art, the gain of the operational amplifier tends to be affected by temperature variations especially in the case of a high gain. Furthermore, it is also apparent that when the hysteresis width .DELTA.V varies, the period of the switching instruction signal SS and consequently the "ON-OFF" period of the transistor 3 varies, so that the operation frequency of the voltage converter also tends to vary.
Thus, the conventional circuit has a problem of maintaining the stable operation as described above, and in addition there is another problem that the hysteresis circuit 9a is complicated in construction, so that its manufacturing cost is expensive in the case of the mass-production of the voltage converters. Furthermore, in the circuit of the type described above, the setting of the hysteresis voltage .DELTA.V which defines the upper and lower limits of voltage control cannot be eliminated because of the operation principle of the circuit. As a result, there has been a strong demand for improving or overcoming the above and other problems encountered in the conventional DC converters.